The damage wrought by Tropical Cyclone Marcus is a "real wake-up call" for Darwin residents lulled into a false sense of security when it comes to cyclone safety, emergency services say.

Key points:

Tropical Cyclone Marcus has left fallen trees and power lines across Darwin

Residents have been urged to stay indoors and off the roads, as hazardous power lines litter the city

People are also advised to boil water, as the extent of contamination is unknown

The category two storm passed over the city just before midday on Saturday, and was downgraded to a category one system about 4:00pm CST.

Darwin saw persistent gale-strength winds of over 62 kilometres per hour for six hours, and recorded a maximum wind gust of 130 kilometres per hour while Marcus was passing overhead.

Authorities said the direct threat of impact from the cyclone had ceased for the city, and conditions were expected to ease overnight.

But NT Police regional controller Warren Jackson said the amount of damage visible across the city from the category two system was a warning of what stronger cyclones could do.

"This is a real wake-up call for Darwin, given that was in what we'd say was the lower end — but it was a category two — you can see there was a significant amount of damage with trees fallen, houses have been damaged … a significant amount of rain has fallen in a short period of time which flooded roads … but as a result of that people still didn't heed our warnings."

A number of people took unnecessary risks during the height of the storm, he said.

"We watched people put themselves in peril today, despite all the warnings in the middle of the storm we saw people driving around into flooded waters, around fallen trees," he said.

"We saw people in the event itself outside of shelters — all they're doing is putting themselves in danger. If they were expecting Emergency Services to come and assist them, that wasn't necessarily going to be the case.

"If you don't necessarily need to travel then reconsider what the purpose of leaving your house or other accommodation is."

He urged residents to stay away from floodwaters, beaches, and waterways.

Hundreds of emergency services personnel would be moving around over the next few days clearing debris and restoring power and water, he said, with the hospital, airport and power and water sites being the priority.

Defence personnel will be helping clear the streets on Sunday.

"We can safely say it will be well into Sunday before we've got a lot of the trees cleared and the power lines made safe, so we need people to really be thinking about what they're doing and do you really need to be going out? If it's not essential, don't go," Mr Collins said.

Survey teams were only a few hours into their work and it was too early to estimate what the damage bill would be, he said.

Two rescues, reports of looting

There were only two main incidents on Saturday and so far no other rescues, Mr Jackson said.

Two people were rescued after a boat broke from its moorings and crashed into their vessel at the Dinah Beach boat ramp.

The NT water police entered the water on jetskis as there were concerns one boat was taking on water. One of the two people on the boat was in their 60s and had a respiratory problem.

At Coolalinga, a tree fell onto a gas cylinder at a petrol station, sparking an evacuation of the surrounding area.

There were reports on social media that the Karama post office and a local car dealership had been broken into.

Mr Jackson said police had not yet had reports of looting, but that it was a concern.

"Crime is always a concern in an emergency situation because people do take advantage of those that are vulnerable … away from their businesses, their houses, whatever the case may be, and it's unfortunate," he said.

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